1. Field of Invention
This invention relates to wireless network such as 802.11 WLANs, and more particularly, to a network interference evaluating method, a dynamic channel assignment method and apparatus used in the wireless network.
2. Description of Prior Art
Wireless networks such as 802.11 WLANs have enjoyed an unprecedented adoption rate in recent years. Generally, the network contains several cells and each cell includes one base station (BS) and many mobile stations (STAs) associated with it. The communications of the STAs must be relayed by the BS. In the context, we use 802.11 WLAN as an example to discuss the issue details, but it shall be noted that the present invention is not intended to be limited in the 802.11 WLAN scenario. In the 802.11 WLANs, the cell is called BSS (Basic Service Set), the BS is called Access Point (AP) and channel refers to wireless frequency spectrum allotted to the WLAN. The 802.11 specification defines a fixed number of channels for use. For example, 802.11b (2.412-2.462 GHz) defines 11 channels.
A fundamental issue in these networks is the interference. Because of the broadcast nature of wireless medium, the transmission from one sender-receiver pair affects those of other pairs. From micro view, the interference amount is relevant with the signal strength. When a packet is sent from a node v to a node u, whether it can be received successfully or not depends on the SINR (Signal to Interference-Noise Ratio) on the interface of the receiver u and the transmission rate of the sender v. Then
                                                                                          SINR                  ⁡                                      (                                          u                      ,                      v                                        )                                                  =                                ⁢                                                      Signal                    ⁢                                                                                                              ⁢                                                                                                            ⁢                    Strength                    ⁢                                                                                  ⁢                    from                    ⁢                                                                                  ⁢                    v                                                                                                                                                            Background                            ⁢                                                                                                                                                      ⁢                                                                                                                                                    ⁢                            Noise                            ⁢                                                                                                                  ⁢                            around                            ⁢                                                                                                                  ⁢                            u                                                    +                                                                                                                                                              Interference                          ⁢                                                                                                          ⁢                          to                          ⁢                                                                                                          ⁢                          u                                                                                                                                                                                            =                                ⁢                                                                            G                                              v                        ,                        u                                                              ·                                          P                      v                                                                                                                          N                        u                                            ⁡                                              [                                                  CH                          ⁡                                                      (                            u                            )                                                                          ]                                                              +                                                                  ∑                                                                              a                            ∈                            V                                                    ,                                                      a                            ∉                                                          AP                              ⁡                                                              (                                u                                )                                                                                                                                                        ⁢                                              I                        ⁡                                                  (                                                      a                            ,                            u                                                    )                                                                                                                                                        ⁢                                  ⁢                              I            ⁡                          (                              a                ,                u                            )                                =                                    G                              a                ,                u                                      ·                          P              a                        ·                          1                              {                                                      CH                    ⁡                                          (                      a                      )                                                        =                                      CH                    ⁡                                          (                      u                      )                                                                      }                                                                        (        1        )            In the Equation (1), Nu[CH(u)] is the background noise of the channel CH(u). a is another sender in the different BSSs, Pv and Pa are the original power level from the senders' network interface. Gv,u and Ga,u are the propagation attenuation factors, which are correlated with the path environments and very hard to compute with the mathematical models. When the SINR becomes worse, the transmitting node needs to lower PHY (physical) rate to prevent any packet corruption. For 802.11 WLANs, the relationship between the PHY rate and SINR is shown in Table 1.
TABLE 1SINR & PHY Data Rate Table802.11 stdPHY Rate (M bps)SINR (dbm)b1−2.92b21.59b5.55.98b116.99a/g1823a/g2425a/g3628a/g4832a/g5435
From macro view, the interference is proportional to the traffic of the interferer because more traffic leads to longer duration of signal interference.
In wireless network optimization, we need to predict interference under different network parameters. For example, if we want to find the best channel assignment of the network, we need to predict interference degrees under different channel allocations and select one with the minimal global interference. The accuracy of interference prediction directly impacts the optimization performance. Similar requirement is needed in other optimizations such as power control and network-diagnosis.
Current estimation of the interference is highly inaccurate.
Most existing works such as Reference [1] define interference based on simple, abstract model of radio propagation—the interference range is twice the communication range. When the receiver is in the interference range of the interferer, the interference between them is 1. Otherwise, it is set into 0.
Authors in Reference [2] provide a method of automatic channel decision and an automatic channel allocation system for access points. In their solution, signal strength between access points is used as the interference amount, which is more sophisticated than Reference [1].
Besides interfering signal strength, the authors in Reference [3] take into account the traffic load of interferers. In their solution, The AP periodically switches into a channel j and listens on the channel j for T ms. Then, AP determines the T_load and T_interference and calculates CS(j) and CI(j), which are the monitored load and interfering energy in the channel j respectively. Finally, AP determines an optimal channel j with the minimal (CS(j)+CI(j)).
In Reference [4], the authors propose a station-aware interference metric—
            W      ⁡              (                              ap            i                    ,                      ap            j                          )              =                                        Num                          ap              i                                ⁡                      (                          ap              j                        )                          +                              Num                          ap              j                                ⁡                      (                          ap              i                        )                                                Num                      ap            i                          +                  Num                      ap            j                                ,where Numapi is the STA number in the BSS of APi, Numapi(apj) is the number of STAs that are associated APi but can hear the beacons from APj; and Numapj is the STA number in the BSS of APj, Numapj(api) is the number of STAs that are associated APj but can hear the beacons from APi.